The movement of transposable elements generates a variety of genetic rearrangements that can profoundly affect the genetic program of their host cells. Our model transposon is phage Mu, whose mechanism of transposition is similar to the integration mechanism of retroviral DNA. The Mu transposase interacts specifically with two sets of DNA sites: att sites at each Mu end which participate in strand exchange, and 'enhancer' sites internal to the ends which are not involved in strand exchange but are essential for synapsis. Transposase interactions with these sites on supercoiled DNA lead to the formation of an extremely stable transposase- DNA synaptic complex responsible for executing Mu DNA cleavage and strand transfer. The catalytically active form of the transposase in this complex is a tetramer. Our goal in this proposal is to understand how the free energy of DNA supercoiling assists in the formation of a functional higher-order protein-DNA complex and how the monomeric transposase assembles into the active tetrameric form. We propose experiments to understand the organization of the active site and that of the two site- specific DNA-binding regions within the transposase. It appears from initial results that substrate binding by the transposase may employ novel DNA recognition motifs. The utilization of higher-order protein-DNA complexes to control a reaction pathway is a feature common toe other processes such as transcription, replication and recombination. The knowledge gained from the study of the DNA transposition reaction should thus provide important insights into other complex biological reactions.